In practically all forms of audio utilization such as radio broadcasting, recording and sound reinforcement, there is a peak level constraint. For example, in AM broadcast there is an inherent 100% modulation limit, in FM broadcast there is a legal deviation limit, in lathe mastering of disc records there is a physical limit due to groove spacing and potential groove-to-groove cross-talk, etc. In sound reinforcement involving a microphone there is an ultimate limit in available acoustic power and thus audience coverage due to the amplifier and speaker system, however the maximum usable acoustic power is often limited by positive acoustic feedback from the loudspeaker(s) to the microphone tending to cause "howl" at a critical power level greatly below the ultimate limit. Acoustic feedback is complicated by room acoustics and frequency-dependent standing-wave patterns.
In these various forms of audio utilization, including in particular stereophonic systems, it is generally desired to maximize the apparent loudness level, as perceived by a listener, as much as possible within the abovementioned limitations.